1. Field of Invention
The present invention relates to wavelength converted semiconductor light emitting devices.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials, and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. III-nitride devices formed on conductive substrates may have the p- and n-contacts formed on opposite sides of the device. Often, III-nitride devices are fabricated on insulating substrates, such as sapphire, with both contacts on the same side of the device. Such devices are mounted so light is extracted either through the contacts (known as an epitaxy-up device) or through a surface of the device opposite the contacts (known as a flip chip device).
FIG. 1 illustrates an example of a III-nitride flip chip device, described in more detail in U.S. Pat. No. 6,650,044. LED 2 includes a first semiconductor layer 10 of a first conductivity type and a second semiconductor layer 12 of a second conductivity type. Semiconductor layers 10 and 12 are electrically coupled to active region 14. Active region 14 is, for example, a p-n junction associated with the interface of layers 10 and 12. Alternatively, active region 14 includes one or more semiconductor layers that are doped n-type or p-type or are undoped. Optional transparent superstrate 16 is disposed on semiconductor layer 10. Contacts 18 and 20 are electrically coupled to semiconductor layers 10 and 12, respectively. Active region 14 emits light upon application of a suitable voltage across contacts 18 and 20. Interconnects 22 and 24 electrically couple contacts 18 and 20 to substrate contacts 26 and 28, respectively. In one implementation, semiconductor layers 10 and 12 and active region 14 are formed from III-nitride compounds such as AlxInyGaz,N compounds, and active region 14 emits blue light at a wavelength of, for example, about 470 nm. Optional transparent superstrate 16 is formed, for example, from sapphire or silicon carbide. Substrate 4 comprises silicon, for example. See U.S. Pat. No. 6,650,044, column 3 lines 40-63.
III-nitride LEDs structures are often grown on sapphire substrates due to sapphire's high temperature stability and relative ease of production. The use of a sapphire substrate may lead to poor extraction efficiency due to the large difference in index of refraction at the interface between the semiconductor layers and the substrate. When light is incident on an interface between two materials, the difference in index of refraction determines how much light is totally internally reflected at that interface, and how much light is transmitted through it. The larger the difference in index of refraction, the more light is reflected. The refractive index of sapphire (1.8) is low compared to the refractive index of the III-nitride device layers (2.4) grown on the sapphire. Thus, a large portion of the light generated in the III-nitride device layers is reflected when it reaches the interface between the semiconductor layers and a sapphire substrate. The totally internally reflected light must scatter and make many passes through the device before it is extracted. These many passes result in significant attenuation of the light due to optical losses at contacts, free carrier absorption, and interband absorption within any of the III-nitride device layers. The use of other growth substrates with an index of refraction that more closely matches that of the III-nitride material may reduce but generally will not completely eliminate the optical losses. Similarly, due to the large difference in index of refraction between III-nitride materials and air, elimination of the growth substrate also will not eliminate the optical losses.